The mechanisms that cause early failure in small arterial bypass grafts are not well understood, although a combination of mechanical, hemodynamic, and thrombogenic factors are thought to be involved. Previous work has shown that thrombosis is related to a mismatch in compliance between the graft and the host artery. A thorough evaluation of the effects of compliance and other design parameters is linked to a better understanding of the failure process, which requires an ability to monitor the deposition of thrombus onto the graft wall as a function of time. The objectives of this study are to examine the dynamics of thrombus deposition in a graft, to ascertain the relative sensitivity of graft performance to mechanical and other factors, and to determine if the eventual failure of a graft can be predicted. Synthetic and biological grafts will be implanted in a canine low flow model that is known to produce a significantly higher failure rate in the former. A limited number of experimental parameters will be varied to produce changes in graft performance. Patency will be monitored with implanted miniaturized strain gauges which will also be used to measure the stresses that are generated near anastomoses, where some of the effects of compliance mismatch are produced. Thrombus deposition will be monitored by measuring the uptake of labelled platelets and fibrinogen at the graft wall and will be correlated with patency and other measured variables. After one month, or when failure occurs, luminal surfaces will be examined for mechanical defects by scanning electron microscopy and for sites of lipid deposition by internal reflections spectroscopy. A comparison of the sequence of events in the two basically different types of grafts should highlight the major factors that contribute to early failure and should lead to the design of a more effective synthetic graft.